It has been estimated that in the United States alone that almost five million people suffer from congestive heart failure. Statistics from the American Heart Association also suggest that new cases of heart failure are diagnosed at a rate of about 500,000 each year. Of the newly diagnosed patients fifty percent are likely to die within five years from the initial diagnosis. Of course, these numbers do not account for the number of patients in other countries who also suffer from heart failure. Given these numbers, it is clear that congestive heart failure is a significant human crisis.
Heart failure is a condition that is characterized by a reduced ability of the heart to circulate blood through the body. Typically, an underlying disease, such as high blood pressure (e.g., hypertension), clogged arteries (e.g., coronary artery disease), heart defect (e.g., cardiomyopathy, or valvular heart disease) or some other problem (e.g., diabetes, hyperthyroidism, or alcohol abuse) will lead to a decrease in circulation over time. As the heart works less efficiently, its capacity to circulate blood decreases and the body's requirements for oxygen are not met. The cardiac muscle tends to enlarge as the heart works harder over time to compensate for the decrease in efficiency.
Heart failure may be identified by the phase of the heart cycle that is particularly associated with the nature of the circulatory problem. By this identification, two types of heart failure are known as systolic and diastolic. In systolic heart failure, the cardiac muscles' ability to contract decreases. This loss of contraction results in a decrease in the ability of the heart to force blood through the circulatory system of the body. In diastolic heart failure, the cardiac muscles' ability to relax diminishes. As the heart muscles become stiffer, the heart does not sufficiently fill with blood and thus each subsequent contraction circulates a lower volume of blood.
Alternatively, heart failure may be characterized by whether it stems from problems with the left or right side of the heart. Left-sided heart failure occurs when the left ventricle does not sufficiently pump oxygenated blood to the body. Right-sided heart failure occurs when the right ventricle does not pump adequately, which leads to fluid build-up in the veins.
Although the phrase “congestive heart failure” is often used to describe all types of heart failure including the above listed types, congestive heart failure is more accurately descriptive of a symptom of heart failure relating to pulmonary congestion or fluid buildup in the lungs. This congestion is more commonly a symptom of systolic and left-sided heart failure. As the efficiency of the pulmonary system declines, increased blood volume near the input side of the heart changes the pressure at the alveolar arterial interface, an interface between the lung capillaries and the alveolar space of the lungs. The change in pressure at the interface causes blood plasma to push out into the alveolar space in the lungs. Shortness of breath (“dyspnea”) and general fatigue are typical perceived manifestations of congestive heart failure.
Congestive heart failure (“CHF”) is currently classified by severity. Class I patients have no apparent symptoms and no physical activity limitations. Class II patients experience some symptoms during moderate to severe physical activity. Class III patients suffer symptoms at mild levels of physical activity. Class IV patients experience symptoms with any form of physical activity as well as at rest.
While the only cure to CHF is heart transplant, there are a number of drug and surgical treatments directed at reducing the underlying problem that led to the heart failure and/or to alleviate the symptoms of heart failure. For example, the use of diaretics is intended to reduce congestion by depleting the body of fluids. Vasodilators such as ACE inhibitors are used to expand blood vessels and reduce resistance to blood circulation. Beta blockers seek to reduce the work load on the heart by normalizing the rhythm of the heart. Cardiotonic drugs are directed at increasing the force of the heart's contractions. Surgical procedures include physical manipulation in an attempt to increase the internal size of constricted arteries, for example, by balloon angioplasty or stenting.
As previously noted, as a consequence of heart failure there is a decreased flow of oxygen in the circulatory system. This decease in oxygenated blood through the body has an impact on the body's respiratory controller. Thus, there appears to be a relationship between congestive heart failure and respiratory conditions known as Sleep Disordered Breathing (“SDB”). For example, it has been noted that 50-60% of heart failure patients have SDB. In this category of patients, approximately 29% may be classified as suffering from obstructive sleep apnea, a breathing condition associated with the cessation or limitation of airflow due to occlusion usually at the level of the tongue or soft palate. In addition, 33% of the patients may be classified as suffering from (a) Cheyne-Stokes respiration, a breathing condition characterized by waxing and waning breathing patterns or (b) central sleep apnea, a condition involving a cessation of airflow due to a cessation of patient respiratory effort. For those patients suffering from Cheyne-Stokes breathing, there is a greater degree of concern. These patients have a higher mortality rate then heart failure patients without Cheyne-Stokes breathing.
Sleep disordered breathing has long been treated by application of Continuous Positive Airway Pressure (“CPAP”). CPAP was invented by Sullivan and taught in U.S. Pat. No. 4,944,310. That patent describes continuous positive airway pressure being applied to a patient, through the patient's nares, to treat breathing disorders, including obstructive sleep apnea. It has been found that the application of pressure which exceeds atmospheric pressure, typically in the range 4 to 15 centimeters of H2O, is useful in treatment. The pressure acts as a pneumatic splint to maintain upper airway patency to ensure free flow of air while the patient sleeps.
In one form, nasal CPAP treatment of Obstructive Sleep Apnea (“OSA”) involves the use of an automated blower, such as the AUTOSET T™ device or AUTOSET SPIRIT™ available from ResMed Ltd., to provide a constant supply of air or breathable gas at pressures in the range of 4 to 20 cm H2O to the airway of a patient via a mask. Examples of suitable nasal CPAP masks are the MIRAGE™ nasal mask and the MIRAGE™ full face mask also available from ResMed Ltd. The AUTOSET T™ device continuously monitors the state of the patient's airway and determines an appropriate pressure to treat the patient, increasing it or decreasing it as necessary. Alternatively, bilevel pressures are delivered to the patient as in the VPAP II™ devices also available from ResMed Ltd. Some of the principles behind the operation of the AUTOSET T™ and VPAP II™ devices are described in U.S. Pat. No. 5,704,345. The entire disclosure of U.S. Pat. No. 5,704,345 is incorporated herein by reference.
One form of pressure treatment is delivered in accordance with a smooth pressure waveform template and a continuous phase variable to provide comfortable pressure support substantially in phase with the patient's respiratory cycle. The device is the subject of International Publication No. WO 98/12965. The device is also the subject of U.S. patent application Ser. No. 08/935,785, now U.S. Pat. No. 6,532,957, the entire disclosure of which is hereby incorporated by reference.
Another form of pressure treatment is directed at treatment of Cheyne-Stokes breathing. In a device designated AUTOSET CS™, also provided by ResMed Ltd., pressure support is varied in phase with patient respiration in such a manner to oppose the waxing and waning changes in patient respiration that characterize Cheyne-Stokes breathing. The device is the subject of International Publication No. WO 99/61088. The device is also the subject of a U.S. patent application Ser. No. 09/316,432, now U.S. Pat. No. 6,532,959, the entire disclosure of which is incorporated herein by reference.
At present, there are no known devices with features designed to treat a range of symptoms of heart failure patients through application of pressure as opposed to devices that might only incidentally provide such benefits. U.S. Pat. No. 5,794,615 teaches a device to provide a level of pressure support to reduce cardiac pre-load and after load. However, the device is only taught to provide one continuous level of pressure during inspiration and another level during expiration. The patent does not suggest the provision of a waveform of cardiac pressure oscillations in phase with a patient's cardiac rhythm, a feature of the present invention as described below. Moreover, the device provides no assistance directed to alleviating Cheyne-Stokes breathing or distinguishing between obstructions due to vocal cord reflex and obstructions from typical OSA due to collapse of the upper airway during sleep.
Any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates, at the priority date of this application.